Prior to the present invention, the galvanizing procedure whereby steel strip is both heat treated and metal coated has become well known and highly developed. Generally a cold rolled steel sheet is heated into the intercritical regime (between Ac1 and Ac3) to form some austenite and then cooled in a manner that some of the austenite is transformed into martensite, resulting in a microstructure of ferrite and martensite. Alloying elements such as Mn, Si, Cr and Mo are in the steel to aid in martensite formation. Various particular procedures have been followed to accomplish this, one of which is described in Omiya et al U.S. Pat. No. 6,312,536. In the Omiya et al patent, a cold rolled steel sheet is used as the base for hot dip galvanizing, the steel sheet having a particular composition which is said to be beneficial for the formation, under the conditions of the process, of a microstructure composed mainly of ferrite and martensite. The Omiya et al patent describes a galvanized dual phase product.
According to the Omiya et al patent, a dual phase galvanized steel sheet is made by soaking the cold rolled steel sheet at a temperature of 780° C. (1436° F.) or above, typically for 10 to 40 seconds, and then cooling it at a rate of at least 5° C. per second, more commonly 20-40° C. per second, before entering the galvanizing bath, which is at a temperature of 460° C. (860° F.). The steel, according to the Omiya et al patent, should have a composition as follows, in weight percent:
Carbon:0.02-0.20Aluminum:0.010-0.150Titanium: 0.01 maxSilicon: 0.04 maxPhosphorous:0.060 maxSulfur:0.030 maxManganese: 1.5-2.40Chromium:0.03-1.50Molybdenum:0.03-1.50with the provisos that theamounts of manganese, chromium and molybdenum should havethe relationship:3Mn + 6Cr + Mo:8.1% max, andMn + 6Cr + 10Mo:at least 3.5%
The Omiya et al patent is very clear that an initial heat-treating (soaking) step is conducted at a temperature of at least 780° C. (1436° F.). See column 5, lines 64-67; col 6, lines 2-4: “In order to obtain the desired microstructure and achieve stable formability, it is necessary to heat the steel sheet at 780° C. or above, which is higher than the AC1 point by about 50° C. . . . Heating should be continued for more than 10 seconds so as to obtain the desired microstructure of ferrite+austenite.” The process description then goes on to say the steel sheet is cooled to the plating bath temperature (usually 440-470° C., or 824-878° F.) at an average cooling rate greater than 1° C./second, and run through the plating bath. After plating, cooling at a rate of at least 5° C./second will achieve the desired microstructure of predominantly ferrite and martensite. Optionally, the plated sheet may be heated prior to cooling, in an alloying procedure (often called galvannealing) after metal coating but prior to the final cooling.
Omiya et al clearly do not appreciate that it is possible to achieve a dual phase product without the high temperatures of their soaking step, or that a particular holding step following a lower temperature soak can facilitate the desired microstructure formation.